Lead-acid batteries are a well known source of energy used in a variety of applications including, for example, automotive starting. The central structural elements of conventional lead-acid batteries are positive and negative grids coated with an active material to form plates, each plate having a lug and being separated from adjacent plates within a battery by porous separators. The grids serve as framework and electrical contact between the positive and negative active materials which generally serve to conduct current. This conjoint electrochemical (corrosion) action and structural (load-bearing) role cause stress to the grids, particularly the positive grids. Failure of the battery occurs when the grids are no longer able to provide adequate structural support or current flow. Therefore, the primary properties of interest in grid formation and design are strength and resistance to both corrosive and mechanical stresses. Other properties to consider include castability, compatibility with the active material (adherence), and various electrochemical and metallurgic effects. With respect to the latter properties, fluidity and resistance to "hot-tearing" (shrinkage tearing) upon solidification and grain formation are of primary importance.
Modernly, a large percentage of the battery grids used in commercially-available lead-acid batteries are manufactured by a process generically referred to as "continuous casting" (con-cast). Traditionally, continuous casting machines include a rotary drum having a reticulated grid pattern (i.e., mold cavity) machined into its outer peripheral surface, and a stationary shoe which overlays the grid pattern. The shoe functions both to dispense the molten lead into the patterned mold cavity and to scrape off any excess molten lead upon rotation of the drum. Due to rapid solidification of the molten lead, a continuous grid strip is removed from the drum upon rotation past the shoe. One example of a conventional continuous casting machine and the lead con-cast process associated therewith is disclosed in U.S. Pat. No. 4,349,067 issued to Wirtz et al.
Unfortunately, conventional continuous casting machines suffer from several drawbacks which significantly limit their production capabilities. First, due to the rapid solidification characteristics of molten lead, large temperature gradients can occur in the molten lead flowing through the shoe and, in turn, as it is delivered across the width of the patterned mold cavity. To avoid such undesirable temperature gradients, the width of the shoe has to be relatively narrow which limits the width of the grid strip, thereby impacting the productivity (i.e., grids/min.) of the continuous casting machine. In addition, conventional con-cast processing has difficulty in consistently forming battery grids, particularly positive battery grids, with the desired mechanical strength and resistance to mechanical and corrosive stresses. In particularly, the current con-cast process is considered impractical to form positive battery grids because of the variable metallurgic effects resulting in improper grain formation which lead to increased corrosion and mechanical stress on the grid. Positive grids differ from negative grids basically in that positive grids require greater strength due to anodic attack, and thus are generally formed with an increased cross-sectional thickness in comparison to negative grids. However, increased mold depth is not conducive to con-cast processing because grain formation is disrupted due to the gradient cooling and flow turbulence caused during discharge of the molten lead. As known, poor grain formation leads to increased corrosion, decreased strength, disruption of the reticulum and, eventually battery failure. Thus, recognized deficiencies exist in the field of con-cast processing of lead battery grids which, to this point, have not been adequately addressed.